Ok, I've found the answer I'm looking for.

Copy/paste

Arthur Dalton E320 16 PIN DTC Charts inc. pin 8, pin 14

If the car has a Built-in led/sw , it only address the DM module.. [ Diagnostics Module] ..and that module [ N/59] is nothing more than an Emmssions Monitoring/ Fault Detscting Module . If that module detects an emmission fault , you can go get that basic info with the built-in button.

Any of the actual Engine Mangement and chassis running gear modules [ HFM/EA/CC/ASR/ Etc]... have to be gotten from the other sockets and require a scanner or , at the least , the home-made Led/Sw tool.

The only reason for the DM and the CE lamp is to notify a warning that an Emmissions mandate fault is present in the cars emmisions system and it will not pass an emmissions testing and should be addressed..........
The cause of that fault can come from one of the other management systems , but if other faults in the other modules will not effect the emmissions standards, it will not show up in either the CE lamp or the built-in led.
So, if one does have a couple of codes w/PB , they want to further investigate the cause/s by going into the other modules.
An example would be a code on the DM that simply states "Igniton Fault'... ...b/c it has talked to the HFM module and the DM knows that a mis-fire in the Ignition will effect the cars Emmssions...now , by going into the HFM module, you can get a more specific ignition fault with the Flash tool, [ like coil # T3 output ] and if you actually had a HHT digital scanner , you could get an even more refined Ignition fault specific.
These late OBD1 systems do not get the credit they rightfully are due, and although the were crude in comparison to the OBD2 generation, they cetainly were cool for that time...and I have personally seen hundreds , [ if not thousands] of broken cars right here on this Forum in my 10 years that got fixed with the aid of that diagnostics capability..even just the built-in LED gave some direction, never mind the Homemade one or a digital scanner.
As one looks back, it was crude , but at that time , it was the balls....

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1990 300E
124.030
chassis 103.983